This invention relates generally to the tracking, collection and concentration of sunlight for the purpose of solar energy utilization. Specifically, the invention pertains to mechanical means for effecting two-axis tracking of an array of point-focus Fresnel lens concentrators. In its primary intended application, the mechanism would function as a component of a core daylighting system (which supplies daylight to core building spaces) wherein each lens would focus sunlight into a fiber optic element. The invention may also find use for concentrator photovoltaic systems, and it could possibly have potential for photothermal energy conversion as well.
A wide variety of two-axis tracking mechanisms are known in the prior art. Core daylighting systems use mechanisms such as the pedestal-mounted heliostat mirror in TIR Systems' collector ["Down the Tubes: Solar Light Goes Indoors"; Stephen P. Schuber, Architectural Lighting; Feb. 1987, pp. 32-35] and the fork-mounted Fresnel lens array in the Himawari system ["Catching Some Rays: New Device Delivers Filtered Sunlight Indoors"; Stephen Mallery, Architectural Lighting; Oct. 1987, pp. 27-30]. Examples of photovoltaic trackers include the pedestal-mounted torque tube configuration of the Intersol array and the end-post tilt/roll system used by ENTECH ["Photovoltaic Concentrator Array Evaluation"; A. B. Maish, INTERSOL 85, pp. 1649-1653].
The optical concentration attained by a concentrating collector is fundamentally limited by the system's optical aberrations and mechanical tracking errors. Photovoltaic systems achieve concentration levels of up to around 1000.times., but a core daylighting system employing fiber optics may need to achieve geometric concentrations of up to nearly 10,000.times. in order to minimize the quantity and cost of optical fiber material required. This level of concentration could not likely be achieved with photovoltaic-type systems, largely due to the effect of wind loading which induces large tracking errors.
Core daylighting systems avoid wind loading effects by housing the tracking system under a transparent protective cover such as the Himawari's acrylic globe or TIR Systems' solarium enclosure. The Himawari's enclosure is exceedingly expensive due to the high cost of the acrylic material, and its curved shape introduces optical aberrations, but these disadvantages are avoided with an enclosure system such as TIR Systems' which uses conventional low-cost, flat window glazing material in its construction.
In addition to eliminating wind-induced tracking errors, a collector enclosure isolates the tracking system from severe storm conditions and prevents degradation from long-term environmental exposure. Despite these advantages, it is not practical to enclose conventional photovoltaic collector systems due to their large dimensional scale. Thus, an essential practical feature of a system such as TIR Systems' collector is its relatively small scale.
One disadvantage of TIR Systems' collector is that it requires two reflective optical elements, one for tracking and one for concentrating. Moreover, in comparison to Fresnel lenses a reflective element's optical performance is very sensitive to surface warpage and tilt errors, making it difficult to achieve high optical concentration. Generally, Fresnel lenses are preferred over mirrors for photovoltaic systems and for core daylighting systems such as the Himawari which use fiber optics.
A common shortcoming of all of the tracking systems discussed above is the intrinsic mechanical instability of their tracking drives. A tracking element's rotational position about its tracking axis is constrained by a drive coupling (e.g., a gear mesh) which is conventionally located very close to the tracking axis, making the system very sensitive to torque loading about the axis. A mechanism such as TIR Systems' heliostat tracker which has very short tracking axes is also susceptible to torque loading about axes transverse to the tracking axis. Moreover, these types of tracking systems are also very sensitive to positional errors in the drive coupling (e.g., backlash), making it difficult to achieve stringent tracking tolerances even in the absence of torque loading.